Current medical imaging techniques typically employ standard X-ray tubes. However, such X-ray tubes can pose several limitations on image quality including large focal spot, broad spectral range, inadequate output and long radiation duration.
The diagnostic range of X-rays in medicine typically requires energies from 10-100 keV. A major problem with typical current X-ray emission techniques in medicine is the broadband energy spectrum that contributes to patient dose while not improving the resulting image.
The interaction of a short-pulse, ultra-intense laser with a solid produces hot, dense plasmas, referred to as laser produced plasmas (LPPs). Heating of the target ionizes the surface layer resulting in suprathermal electrons. Some of these electrons are accelerated forward and penetrate into the unperturbed portion of the target. The resulting K-shell ionization leads to the emission of K-alpha X-rays.
While x-ray energy depends on the target material, total X-ray yield and energy spectrum are influenced by laser intensity, contrast and pulse duration as well as target thickness and geometry. LPP X-rays sources typically have a small source size due to the rapid heating of the target. Furthermore, the, typically, picosecond duration of the X-ray emission coupled with a high repetition rate laser can produce a high fluence of X-rays in a very short period of time.
Metal covered targets are used in some high energy physics applications, such as inertial confinement fusion. In some cases, such targets are shot with a laser in order to generate plasmas or high energy radiation.
Targets commonly used with lasers to produce plasma and radiation can suffer from several disadvantages. For example, conventional targets are often produced by micro-machining processes that typically produce targets having a tip sharpness, or apex dimensions, of 25 μm or larger. For example, an existing process involves micro-machining a mandrel, electroplating the mandrel with a desired metal, and then etching away the mandrel. Other processes involve depositing a metal layer on a plastic mold and then melting away the plastic mold. Some prior experiments have used metal coated silicon targets. However, the silicon included in such targets typically interferes with energy focusing and radiation enhancement.
The tips of targets produced by such processes can be significantly larger than the wavelength of the laser light that will be used with the target and therefore may not produce optimal energy. Similarly, the apexes, or tips, of the targets can be larger than the focal size (or spot size) of the laser, which can minimize any enhancements that might otherwise be conferred by the target shape.
In addition, such targets are typically manufactured individually and thus can be comparatively expensive. The expense of the targets may limit the number of targets available for use, thus potentially limiting how the targets can be used. For example, a limited number of targets available for a series of experiments may limit the quality or quantity of data obtained during the experiments.
The amount of material available on such targets, or irregularities in the target surface, may interfere with full characterization of the produced plasma. Insufficient target material may also interfere with optimal energy production.
While hemispherical laser targets have been tested, such targets typically suffer from disadvantages in addition to those noted above. For example, irregularities in the surface of the target, or variations in the targets resulting from their method of manufacture, may make it difficult to properly position the target and position other objects with respect to the target.